Problem: On the first 3 tests of his history class, Christopher got an average score of 80. What does he need on the next test to have an overall average of 84?
Let his score on the next test be $x$ The sum of all of his scores is then $3 \cdot 80 + x$ The same sum must also be equal to $4 \cdot 84$ Solve: $x = 4 \cdot 84 - 3 \cdot 80 = 96$.